Water-absorbing resins have found wide use in sanitary goods, hygenic goods, wiping cloths, water retaining agents, dehydrating agents, sludge coagulants, disposable towels and bath mats, disposable door mats, thickening agents, disposable litter mats for pets, condensation preventing agents and release control agents for various chemicals. Water-absorbing resins are available in a variety of chemical forms including substituted and unsubstituted natural and synthetic polymers such as hydrolysis products of starch acrylonitrile graft polymers, carboxymethylcellulose, cross-linked polyacrylates, sulfonated polystyrenes, hydrolyzed polyacrylamides, polyvinyl alcohols, polyethylene oxides, polyvinylpyrrolidines and polyacrylonitriles.
In some polymerization processes for manufacturing water-absorbent polymers, such as cross-linked polyacrylic super-absorbent polymers, e.g., polyacrylic acid, or partially neutralized or fully neutralized polyacrylic acid, one or more monomers are polymerized in water to produce a polymer that then must be ground to provide a desired particle size, with or without an intermediate drying step, for incorporation into a variety of different products, as outlined above. During the grinding process, fine particles result that are undesirable due to dusting problems, or other manufacturing problems. Fine superabsorbent material is considered to be undesirable in many personal care applications including infant diapers and adult incontinence devices. Such fine material can migrate in the device before use and exhibit gel blocking in application.
European Patent EP 0 463 388 A1 (Hoescht Celanese) discloses a process for recycling sub 75 .mu.m SAP fines back into a reaction gel at 16%-17% solids. However, only 4% recycle is possible and the addition of extra process water is required. International classification CO8L33/02 (Seitetsu) describes a method whereby fine powder may be blended with a prepared polyacrylate solution into a crumbly mix which generates agglomerates on drying at up to 150.degree. C. International publication number WO 90/08789 (Dow) describes an agglomeration route involving the use of hydrocarbon solvents to suspend fine particles which are then clustered by the addition of acrylate monomer solution under polymerization conditions in the presence of an amorphous silica powder. Seitetsu also disclose the use of organic solvents as a dispersion phase in U.S. Pat. No. 4,732,968 (EP 0 224 923). SAP fines are dispersed in an inert solvent with addition of silica in the presence of water and a suitable surfactant, followed by removal of solvent and drying.
All the above disclosures share disadvantages of either relatively low rates of fines consumption, or the use of large amounts of organic solvents and expensive silica additives. In addition there is no claim of enhanced superabsorbent performance other than lack of gel blocking in the agglomerated material.
In the present invention, a preferred process is disclosed wherein fine SAP particles are surface treated with an aqueous solution of cross-linking agent, preferably applied as a fine mist to freely tumbling SAP particles. The surface coated particles then are subjected to an elevated temperature, in order to increase the reaction rate of the surface cross-linking reaction, and to dry the coated particles to a moisture content of about 15% by weight or less. In the preferred embodiment, to achieve an increased particle size, the dried product of this surface cross-linking stage then is mixed intimately with a suitable amount of water, e.g., in a high shear mixer, such as a Z-blade mixer or Sigma blender, in order to achieve a continuous paste. Preferably, the paste is subjected to conditions which simultaneously promote drying of the paste particles. Upon subsequent milling of the dry product (0 to about 15% by weight water based on the dry weight of the polymer), particles of increased particle size are obtained. These particles retain particle integrity upon hydration and demonstrate enhanced polymer performance in the absorption under load test, described in more detail hereinafter.